Abstract

It has been hypothesized that the ocean mesoscale (particularly ocean fronts) can affect the strength and location of the overlying extratropical atmospheric storm track. In this paper, we examine whether resolving ocean fronts in global climate models indeed leads to significant improvement in the simulated storm track, defined using low level meridional wind. Two main sets of experiments are used: (i) global climate model Community Earth System Model version 1 with non-eddy-resolving standard resolution or with ocean eddy-resolving resolution, and (ii) the same but with the GFDL Climate Model version 2. In case (i), it is found that higher ocean resolution leads to a reduction of a very warm sea surface temperature (SST) bias at the east coasts of the U.S. and Japan seen in standard resolution models. This in turn leads to a reduction of storm track strength near the coastlines, by up to 20%, and a better location of the storm track maxima, over the western boundary currents as observed. In case (ii), the change in absolute SST bias in these regions is less notable, and there are modest (10% or less) increases in surface storm track, and smaller changes in the free troposphere. In contrast, in the southern Indian Ocean, case (ii) shows most sensitivity to ocean resolution, and this coincides with a larger change in mean SST as ocean resolution is changed. Where the ocean resolution does make a difference, it consistently brings the storm track closer in appearance to that seen in ERA-Interim Reanalysis data. Overall, for the range of ocean model resolutions used here (1° versus 0.1°) we find that the differences in SST gradient have a small effect on the storm track strength whilst changes in absolute SST between experiments can have a larger effect. The latter affects the land–sea contrast, air–sea stability, surface latent heat flux, and the boundary layer baroclinicity in such a way as to reduce storm track activity adjacent to the western boundary in the N. Hemisphere storm tracks, but strengthens the storm track over the southern Indian Ocean. A note of caution is that the results are sensitive to the choice of storm track metric. The results are contrasted with those from a high resolution coupled simulation where the SST is smoothed for the purposes of computing air–sea fluxes, an alternative method of testing sensitivity to SST gradients.

Notes

Acknowledgements

An anonymous reviewer is thanked for constructive comments on the paper, especially for suggesting a more detailed assessment against ERA-Interim Reanalysis. CESM1 was a community development effort, as described in Hurrell et al. (2013), for which we are very grateful. The CESM1 simulations described in this paper were performed at NCAR Wyoming Supercomputer Center. We also thank Tom Delworth, Gabriel Vecchi and all the scientists at GFDL who invested time and resources to produce the GFDL CM simulations used in this research. RJS was supported by Department of Energy Office of Biological and Environmental Research, via the Scientific Discovery through Advanced Computing (SCIDAC) project number SC0006743,and National Science Foundation (NSF) Collaborative Research: EaSM-3: Regional decadal predictions of coupled climate-human systems 1419585. Y-OK was supported by NSF Division of Atmospheric and Geospace Science Climate and Large-scale Dynamics Program (AGS-1355339), NASA Physical Oceanography Program (NNX13AM59G), and DOE Office of Biological and Environmental Research Regional and Global Climate Modeling Program (DE-SC0014433). JFB was partially supported by the National Oceanic and Atmospheric Administration (NOAA) Climate Program Office’s Modeling, Analysis, Predictions, and Projections program, Grant #NA15OAR4310094. R. Msadek was supported by GFDL and UCAR when the GFDL simulations used in this paper were produced and analyzed. We thank Steve Griffies for helping interpreting the results. Discussions with Hisashi Nakamura, Niklas Schneider and Shoshiro Minobe were very appreciated.